1. Field of the Invention
This invention relates to a signal processor circuit for use individually in conjunction with each pixel of a focal plane array (FPA) to improve the signal to noise ratio of the output of each pixel of the array.
2. Brief Description of the Prior Art
Focal plane arrays and particularly focal plane staring arrays are composed of a plurality of infrared detector elements or pixels in a matrix arrangement (e.g., 64 pixels.times.64 pixels) which are generally cooled to a temperature in the range of liquid nitrogen (77.degree. K.). The array is stationary so that each element of the array continually stares at the same portion of the scene. Since nothing is changing, advantage of this fact is taken by performing integration over some period of time, it being apparent that the longer the integration, the more the signal will build with respect to the noise. This is due to the fact that the signal level is multiplied by the amount of time that integration takes place whereas the noise is multiplied by the square root of time and is therefore smaller, thereby improving the signal to noise ratio.
Each detector element includes a capacitor which accumulates charge thereon from a portion of the scene under observation, the time accumulation of charge resulting in an integration. Both signal and noise current are integrated onto the capacitor. It is desirable that the capacitors be large so that they can hold more charge and permit integration over extended periods of time as indicated above. However, the capacitance of capacitors is area dependent, so large value capacitors require large device area relative to the size of each pixel and cannot be easily accommodated as a part of the array. For this reason, the capacitors are generally very small and accommodated as part of the array.
Since the capacitor charging signal current can be rather large, it follows that the small capacitance value capacitor can only integrate for a given possibly relatively short time before the voltage across the capacitor becomes so large that the capacitor can no longer handle further accumulated charge thereon. This means that a substantial portion of the potential charging time is not used and is wasted. This provides uncertainty in the result of the integration.
It is desirable that integration of a signal at each pixel of a detector which is focussed upon a given small area of a scene take place for a period as long as possible and, in any event, over an extended period of time so that the noise provides a relatively small amount of the total signal and thereby diminishes its effect thereon.